1. Field of the Invention
The present invention relates to a broadcast communication system for distributing electronic mail to a plurality of specific subscribers over networks such as an internet and a local area network (LAN) and an electronic mail distribution method thereof.
2. Description of the Related Art
As a mode of using electronic mail by which data and messages are sent or received to/from a plurality of computer systems such as a personal computer and a workstation connected through a network, there is broadcast communication which distributes electronic mail to a plurality of specific subscribers at a time. In recent years, with the development of a public wide area network (WAN) such as an internet, use of broadcast communication by means of electronic mail has been sharply rising.
Conventionally employed for a broadcast communication system using electronic mail of this kind are an electronic mail system shown in FIG. 6 and a mail server shown in FIG. 7. FIG. 6 is a schematic diagram showing structure of an electronic mail system, while FIG. 7 is a block diagram showing structure of a conventional mail server which realizes an electronic mail system.
The electronic mail system shown in FIG. 6 has a mail server 600, a sending station 740 for sending electronic mail and a plurality of accepting stations 750 for receiving electronic mail connected with each other over a network. In the illustrated example, the mail server 600, the sending station 740 and each accepting station 750 are respectively connected to individual networks 231, 232 and 233 which are connected to an internet 210 through access points 221, 222 and 223, respectively. This arrangement enables transmission of electronic mail from the sending station 740 to each accepting station 750. Although the sending station 740 for sending electronic mail and the accepting station 750 for receiving electronic mail are separated from each other for the explanation's sake, they have no difference in structure as a terminal unit and function as the sending station 740 when individual terminal units connected to the network send electronic mail and function as the accepting station 750 when they receive electronic mail. Also in the illustrated example, although the mail server 600 and the terminal units 740 and 750 are connected to a plurality of networks connected to each other over the internet 210, networks can take any form and even with a simple WAN or LAN not connected to the internet 210, basic structure of an electronic mail system including the mail server 600, the terminal units 740 and 750 remains the same.
With reference to FIG. 7, the mail server 600 includes an input/output interface 610, an input unit 620 and an output unit 630 for sending and receiving electric mail and other data to and from a network 231, a storage unit 640 for storing electronic mail, a command processing unit 650 for conducting various processing for the distribution of electronic mail, and a control unit 660 for controlling operation of each of these function execution units.
In thus structured electronic mail system, a user who intends to send electronic mail (hereinafter referred to as sender) sends a set of mail data containing a main body (text) of electronic mail which is a main portion of the electronic mail, transmission destination data indicating a destination of electronic mail, and a transmission command indicative of an electronic mail distribution condition from the terminal units 740 to the mail server 600. Mail data applied to the input/output interface 610 of the mail server 600 through the network 231 is transferred to the input unit 620 by the instruction from the control unit 660. Then, of the mail data in question, the electronic mail main body and the transmission destination data are transferred to the storage unit 640 and stored at a predetermined position. The transmission command is transferred to the command processing unit 650.
Next, the command processing unit 650 analyzes the received transmission command and in accordance with the instruction of the transmission command, instructs the storage unit 640 to retrieve and output an electronic mail main body and transmission destination data. The storage unit 640 transfers the electronic mail main body and the transmission destination data to the output unit 630 in accordance with the instruction from the command processing unit 650. The output unit 630 sends the electronic mail main body in question to the accepting station 750 used by a predetermined user (hereinafter referred to as receiver) in accordance with the transmission destination data in question.
In the foregoing processing, designating a plurality of receivers as transmission destination data leads to realization of broadcast communication by means of electronic mail. Depending on the contents of a transmission command, a main body of electronic mail in question may be held in a mail box assigned to a receiver in the storage unit 640 in accordance with instructions from the command processing unit 650 and the control unit 660 and be transmitted in response to access made by the receiver in question by means of electronic mail.
In the above-described broadcast communication using electronic mail, increase in the number of subscribers to service as a party to which electronic mail is sent results in concentration of enormous traffic on a network (the network 231 in the example shown in FIG. 6) to which the mail server connects. This adversely affects throughput of other terminal units connected to the network in question.
In addition, there is a case where information of broadcast-communicated electronic mail is unnecessary temporarily or for a long period of time for reasons of a subscriber to service. In this case, sending electronic mail to the subscriber to the service in question results in wasteful use of network resources to deteriorate efficiency.
One of conventional art intended to efficiently use a network line while avoiding such situations as mentioned above is, for example, a technique disclosed in Japanese Patent Laying-Open (Kokai) No. Heisei 64-11442, entitled "Electronic Mail Communication Control Method". The literature recites a communication control method aimed at reducing contentions among sending stations by divisionally managing electronic mail as a main body of electronic mail and a header part, notifying an accepting station of a header part prior to transmission of an electronic mail main body and preengaging transmission time of an electronic mail main body.
While the communication control method disclosed in the above literature enables prevention of congestion in terms of time at the transmission of electronic mail, the method is still premised that a main body of mail is sent to all terminal units. This is because no means is provided for determining at an accepting station whether information of electronic mail is necessary for a subscriber to service. This disables avoidance of wasteful use of a network line caused by the transmission of electronic mail also to a terminal unit of a subscriber to service who needs no information of the electronic mail.